Known driver stages or power driver circuits for driving the loads include high power transistors. In references, various schemes are known and partially even patented, in which a control of a rounding of a slope of characteristic curves of the concerned transistors or the control of a slew rate of a field-effect transistor, i.e. a drain/source voltage Vds of the field-effect transistor of the output stage as compared to a charge of the gate electrode of the buffer stage are mentioned. These schemes often use various (protectable) buffer circuits with various strengths for controlling their associated output stages. Further known schemes use reference currents (bias currents) generated by bandgap reference sources for controlling the slew rate within their buffer circuits to achieve charging or discharging of, for example, a gate electrode of a field-effect transistor with a constant current. Other techniques measure voltage supply changes and then use a slew rate compensation in their buffer circuits.
The disadvantage of these schemes is that they do not take into account the need for an adapted switching behavior. Particularly the aspect of the duty cycle, i.e. the ratio of a turn-on time divided by a turn-off time, is not taken into account. Neither do these schemes take into account that the output stage including the transistor may be considerably loaded depending on the supply conditions, i.e. may be subjected to considerable stress.
As a further consequence, systems with a high inductive load and free-wheeling diodes, so-called circulation diodes, have considerable deficiencies with respect to target current accuracy and/or charge current accuracy.
An active reference current network is known, which compensates for a change of the reference current caused by hot-carrier injection and representing a common phenomenon in existing metal-oxide semiconductor transistors (MOS transistors) and particularly in laterally diffused MOS transistors (LDMOS). The known active reference network first measures the reference current flowing through a target transistor and compares it to a stable reference current. The difference between the reference current in the target transistor and the reference current is then used to adjust the reference current in the target transistor with a current mirror feedback circuit. The reference current of the target transistor is thus stable and independent of influences by hot-carrier injection on the reference current and other alterations of the reference current due to unfavorable circumstances. A MOS transistor used for measuring the reference current is operated in the linear region (triode region) of its characteristic curve and thus has minimal influence on the performance of the target transistor.